The present invention relates in general to digital waveform synthesizers and in particular to a waveform synthesizer for generating angle modulated waveforms.
Digital waveform synthesizers of the prior art typically generate a sequential set of digital data representing the desired waveform. This data is applied to a digital-to-analog (D/A) converter which then produces the synthesized analog waveform of voltage magnitude varying according to the magnitude of the applied digital data. A typical such waveform synthesizer employs an offset register, an accumulator and a random access memory (RAM) wherein data representing one period of the selected waveform completely fills all RAM locations. When the RAM address is sequentially updated in selected incremental steps, the data appearing at the output terminals of the RAM varies in proportion to the magnitude of the desired synthesized analog waveform. The frequency of output waveform variation is directly proportional to both the address step size and the address update rate (i.e. system clock frequency), and inversely proportional to the RAM length. The address step size is stored in the offset register and is accumulated by the accumulator, which provides the incremental RAM addresses.
While digital waveform synthesizers of the prior art are well adapted for digitally synthesizing waveforms oscillating according to a single frequency, they are not well adapted for synthesizing angle (e.g. phase or frequency) modulated waveforms. Phase and frequency modulated waveforms oscillate according to two independant frequencies, a higher carrier frequency and a lower modulating frequency. In order to produce a phase or frequency modulated waveform, two digital waveform synthesizers, driving associated D/A converters, could be used with one synthesizing a waveform of the carrier frequency and with another synthesizing a waveform of the modulating frequency. The two analog waveforms thus synthesized could then applied to a conventional analog phase or frequency modulating circuit to produce the angle modulated waveform.
This method therefore uses two D/A converters and a conventional analog modulating circuit. The extra D/A converter and the analog modulating circuit can be expensive and can take up a comparatively large amount of space in an equipment chassis. It would be advantageous if data representing a frequency or phase modulated waveform could be sythesized entirely by digital means such that only a single D/A converter is needed to generate the analog waveform equivalent and such that no analog modulating circuit is required.